THE AECHIPELAGO OF CHAUSEY. 6 



wonder-revealing instrument — can penetrate even to 

 the inmost recesses of its organisation.* Then there 

 was the Brachionus, another genus of the class of 

 rotifers, which, on the slightest indication of the ap- 

 proach of danger, covers its long tail and ciliated 

 head with its bristling cuirass. Next in order came 

 some of those Diatomacece] , whose infinitely minute 

 siliceous shields have offered a firmer resistance 

 against the revolutions of our globe than the gigan- 

 tic skeletons of the antediluvial monsters — organisms 

 so microscopically minute that the point of a needle 

 might at one touch crush hundreds of them, although 

 their remains have combined to form entire rocks 

 and extensive geological strata, known and worked 

 for ages under the name of tripoli. Lastly there 

 were Planarias J, and myriads of infusorial animal- 



* Hydatina senta belongs to the class of the Rotifers. It was on 

 this species that Ehrenberg made his first observations on the com- 

 plicated organism of these little animals. The hydatina is of very 

 common occurrence in the neighbourhood of Paris, especially in the 

 spring, when it is to be met with in the little pools of stagnant 

 water on the road- side, and in the ruts made by carriage wheels. 



f The Diatoms constitute one of those groups regarding the 

 position of which naturalists are still undecided ; some holding them 

 to be vegetables, while others regard them as animals. Some of 

 them, as the Naviculas, exhibit a slow and regular motion which 

 appears to be the result of spontaneity. Many present forms of 

 geometrical regularity, and their siliceous shields, transparent as the 

 purest crystal, are moreover, marked with tracings of such extreme 

 delicacy that every improvement in the microscope reveals to us 

 new and previously unobserved details. 



X The Planarias belong to the great subdivision of the Vermes. 



They are flat, slightly elongated animals in which the two sexes are 



united ; they are provided with a digestive apparatus, which ramifies 



over the whole bodj, and they move by the aid of vibratile cilia, 



B 2 



